DESCRIPTION: (Adapted from the Abstract). The potential usefulness and importance of topical in-vivo ESR are cited as the following unique properties which distinguish it from other techniques: a) In-vivo detection of extrinsic paramagnetic species (spin labels) targeted to specific sites in a biological system. b) In vivo monitoring using spin labels which monitor structural order at the microscopic level (membranes, proteins, cell walls, etc.) c) Direct in-vivo detection of free radical species (naturally occurring or "trapped" products from transient free radical reactions). The specific aims of this Research Resource development renewal proposal are: 1. Further development and optimization of low field in-vivo topical ESR spectrometers, including microwave resonators in the low GHz range (500-1000 MHz); assessment of feasibility, sensitivity, power losses, and specifications of a low field topical ESR system. 2. In-vivo monitoring of spin label distribution and metabolism in animals: a. in-vivo pharmacokinetics of nitroxide metabolism and excretion b. monitoring tumour growth characteristics by spin label metabolism c. spin trapping studies of a swine global myocardial ischemia reperfusion model; d. nitroxide monitoring in the brain.